1. Field of the Invention
The present invention relates to a method and apparatus for pass control of transmission cells.
In a broadband integrated service digital network (B-ISDN), cells composed of so-called "fixed length packets" are transmitted. This B-ISDN is constructed by various types of cell processing apparatuses, for example, an asynchronous transfer mode (ATM) exchange, transmission apparatus, cross-connect apparatus, and ATM-local area network (LAN) equipment. The above cells are input to these cell processing apparatuses as transmission cells, subjected to predetermined processing, and then output.
The present invention will discuss the pass control of transmission cells as one of the predetermined processing. Pass control is a control, when the cell processing apparatus detects transmission cells including a bit error or errors, for not allowing such cells to pass through a cell processing apparatus but, for example, discarding them.
2. Description of the Related Art
As will be explained in detail later by using the drawings, in an ATM exchange, ATM cells are extracted at each individual part constituting an input/output division of the ATM exchange. In the extraction of the ATM cells, first cell synchronization is established by a synchronizing field mapped in a header of each ATM cell. A typical synchronizing field is a header error control (HEC) field.
The value written in the HEC field is the result of a cyclic redundancy check (CRC) calculation on the 5 bytes (5 octets) comprising the header. When a specific pattern of a multi-bit error occurs in the header, however, this multi-bit error is erroneously recognized as a one-bit error. Occurrence of such an erroneous recognition is inherent to the above CRC calculations and cannot be avoided. Note that a detailed example of the specific pattern causing the erroneous recognition will be explained later.
When a specific pattern of the multi-bit error occurs, the transmission cells must be discarded without passing through the cell processing apparatus. However, because of the characteristics of CRC calculation, the specific pattern of the multi-bit error is erroneously recognized as one-bit error as mentioned above and the cells pass through the cell processing apparatus. This is because, under an HEC synchronizing algorithm using CRC calculation, a header deemed to include one-bit error therein, regardless of the fact that this is a pseudo one-bit error, is rewritten to a header having a bit corrected at that error bit position, and the transmission cells are then sent again onto the lines.
The problem is that such a cell including an error in its header exerts a greater influence upon the reliability of the communication system than a cell including error in its payload. This is because error included in the payload can be easily recovered by a request for retransmission from the terminal of destination subscribers of the cells, but when the error is included in the header, for example, the cell in question is transferred to the other unrelated subscribers, the cell with a high priority is discarded first at the time of congestion of traffic during transmission, and other similar situations are caused.